1. Field of the Invention
The present invention relates to an image forming method and image forming apparatus, and, more particularly, to an image forming method which collects residual toners on a latent carrier with a developing device to thereby eliminate a cleaner, and an apparatus for accomplishing the same.
2. Description of the Related Art
In image recording apparatuses, such as a copying machine, a printer and a facsimile, a latent image forming apparatus like an electrophotographic apparatus is used due to the popularity of image recording on normal sheets of paper. Such an image forming apparatus will be described below as a typical electrophotographic printer. As shown in FIG. 1A, various types of process units are disposed around a photosensitive drum 1, such as an organic photosensitive body, Se photosensitive body or a-Si photosensitive body. More specifically, arranged around the photosensitive drum 1 are a corona charger 2 for uniformly charging the surface of the photosensitive drum 1, a laser optical system 3 for performing image exposure, a developing device 4, such as a two-component developing device, magnetic one-component developing device or non-magnetic one-component developing device, a corona discharger 5 for electrostatically transferring a toner image on the photosensitive drum 1 onto a paper P, a cleaner 6 such as a fur brush cleaner or a blade cleaner, and a deelectrifying lamp 7. A fixing device 8 for fixing the toner image on the paper P with heat and pressure is further provided on a sheet-conveying passage where the paper P is conveyed.
The image forming operation is performed in the following manner. First, the surface of the photosensitive drum 1 is uniformly charged by the corona charger 2 and then the charged surface is exposed with an optical image corresponding to a target image by the laser optical system 3, thus forming an electrostatic latent image corresponding to the target image. Then, charged toners are supplied from this developing device 4 to the electrostatic latent image on the photosensitive drum 1 to develop the image. The corona discharger 5 as a transfer device is disposed in the vicinity of the photosensitive drum 1 with the paper P in between, and charges the back of the conveyed paper P to the opposite polarity to that of the charges of the toners, thereby electrostatically transferring the toner image on the photosensitive drum 1 onto the paper P. While the paper P carrying this toner image passes through the fixing device 8, the toner image is fixed on the paper P with heat and pressure, completing the printing.
The efficiency of transferring the toner image on a sheet of paper is not 100%, and some toners will remain on the photosensitive drum. Therefore, the top surface of the photosensitive drum 1, after transfer of the toner image onto the paper, P is cleaned with the cleaner 6 to remove the residual toners. Then, the deelectrifying lamp 7 is activated to remove the residual charges on the photosensitive drum 1. The drum 1 is thus returned to the initial state to be ready for another printing operation.
The residual toners collected from the photosensitive drum 1 by the cleaner 6 are temporarily stored in a waste-toner tank by a toner carrying mechanism (not shown), and a user will dispose of this tank when a predetermined amount of waste toners is stored. This image forming process requires a toner disposal mechanism and space for storing the waste toners, and stands in the way of making the image forming apparatus compact. As the toners collected by the cleaner 6 do not contribute to printing, this process is not economical. Further, the toners require special handling during disposal to protect the environment.
In view of the above and due to the recent demands for smaller apparatuses and lower cost, it is desirable to eliminate part of the recording process. As one solution, a cleanerless process to eliminate the need for the cleaner has been proposed as in, for example, xe2x80x9cCleanerless Laser Printer,xe2x80x9d Electrophotographic Institute Report, vol. 30, no. 3, pp. 293-301.
This cleanerless process eliminates the cleaner 6 and allows the residual toners after image transfer to be collected by the developing device 4 so that the residual toners can be used again for printing. As shown in FIG. 1B, the cleaner 6 is eliminated and a conductive uniform brush 9 is provided instead in the cleanerless process. In this recording process, the residual toners on the photosensitive drum 1 are distributed by the brush 9. Then, the surface of the photosensitive drum 1 with toners thereon is uniformly charged by the corona charger 2, an image exposure is performed by the laser optical system 3, and the collection of the residual toners and developing of the electrostatic latent image are carried out simultaneously by the developing device 4.
The toners concentrating on one portion are distributed by the uniform brush 9 to reduce the amount of toners per unit area, thereby facilitating the toner collection by the developing device 4. Further, as the toners are distributed, the residual toners are prevented from becoming a filter for the ion shower from the corona charger 2 to thereby avoid non-uniform charging. Also, the toners in the exposing step are prevented from becoming a filter to thereby avoid uneven exposure.
The point of this recording process is the collection of the residual toners on the photosensitive drum 1 is performed at the same time as the developing step. This point will be described referring to FIG. 2A showing the photosensitive drum 1 charged to a negative potential with toners also charged to a negative potential. The surface potential of the photosensitive drum 1 is set to xe2x88x92500 to xe2x88x921000 V, and the potential of that exposed portion where the potential drop has occurred due to the image exposure is dropped down to minus several tens of volts, thereby forming an electrostatic latent image. At the developing time, a developing bias potential lying nearly in the middle of the surface potential and the latent image potential is applied to a developing roller. In the developing step, the negatively-charged toners on the developing roller stick on the electrostatic latent image on the photosensitive drum 1 by an electric field formed by the developing bias potential and latent image potential, thereby providing a toner image.
In the cleanerless process, at the same time as the developing step is performed, the residual toners after image transfer, distributed over the photosensitive drum 1 by the uniform process, are collected to the developing roller from the surface of the drum 1 by the electric field that is created by the surface potential and the developing bias potential.
This cleanerless process apparently requires that the amount of the residual toners on the photosensitive drum 1 after collection in the developing device 4 be considerably small. Let us now consider the transfer step. When the corona discharger of an ordinary type is used as a transfer device, the charges given to the paper P by the corona discharge leak during periods of high humidity. Accordingly, an electric field may not be applied to the toners on the photosensitive drum 1, reducing the transfer efficiency. More specifically, the normal transfer efficiency of 80 to 90% drops to 50 to 60% at a high humidity. The reduction in transfer efficiency increases the quantity of the residual toners after image transfer. It therefore becomes difficult to completely collect the toners from the photosensitive drum 1 in the developing step, causing background noise to stick toners on the background of the paper. This reduces the print quality.
Let us now turn to the toners. Pulverized toners, which are normally used, have deformed shapes of uneven particle sizes. The pulverized toners therefore have a high mechanical adhesive strength to the photosensitive drum 1, making the toner transfer difficult. In addition, it becomes difficult to provide close contact of the toners with the paper in the transfer section. This weakens the electric field, reducing the transfer efficiency to 80 to 90%. Consequently, the quantity of the residual toners after transfer increases, making it difficult to completely collect the toners from the photosensitive drum 1 in the developing step. This also causes background noise to stick toners on the background of the paper, thus reducing the print quality.
Let us now consider the charging step. Most of the toners which are not transferred in the transfer step are those which have been charged to the opposite polarity. The oppositely charged toners will not be collected according to the principle of collecting the residual toners in the above-described developing step. This apparently causes background noise on the paper. The residual toners receive charged ions produced by the corona charger 2 in the charging step. While the photosensitive drum 1 is charged to a predetermined potential by the charged ions, the oppositely charged toners will not be charged to such a potential. The oppositely charged toners, even if charged by those ions, will keep the opposite polarity. This makes the toner collection difficult in the developing step. In addition, as shown in FIG. 2B, potential irregularity occurs on the surface of the photosensitive drum 1 after the transfer step. Even if the surface of the photosensitive drum 1 is uniformly charged in the charging step, this potential irregularity is likely to remain in that portion where the toners remain. This causes afterimage, background noise and the like, deteriorating the print quality.
It is therefore a primary object of the present invention to provide an image forming method and image forming apparatus, which improve the print quality in the cleanerless process.
It is another object of the present invention to provide an image forming method and image forming apparatus, which improve the transfer efficiency to thereby reduce the quantity of residual toners after image transfer in the cleanerless process. It is a further object of the present invention to provide an image forming method and image forming apparatus, which charge the oppositely charged toners remaining after image transfer to a positive potential and collect the residual toners in the developing step in the cleanerless process.
To achieve the foregoing and other objects in accordance with the purpose of the present invention, according to one aspect of the invention, an image forming apparatus comprises a rotary endless latent carrier; first means for charging the rotary endless latent carrier; second means for forming an electrostatic latent image on the rotary endless latent carrier; third means for developing the electrostatic latent image on the rotary endless latent carrier by supplying polymerization toners simultaneously with cleaning residual polymerization toners on the rotary endless latent carrier; and fourth means for transferring the polymerization toners on the rotary endless latent carrier to a sheet.
An image forming method according to this aspect comprises a first step of charging a rotary endless latent carrier; a second step of forming an electrostatic latent image on the rotary endless latent carrier; a third step of developing the electrostatic latent image on the rotary endless latent carrier by supplying polymerization toners simultaneously with cleaning residual polymerization toners on the rotary endless latent carrier; and a fourth step of transferring the polymerization toners on the rotary endless latent carrier to a sheet.
As the polymerization toners have a smooth shape, the mechanical adhesive strength of attachment to the latent carrier is small. It is therefore easier to transfer the toners on a sheet, thus improving the transfer efficiency. Further, since the polymerization toners have an even particle size, the clearance between the latent carrier and the sheet becomes smaller so that the electric field for image transfer can be applied to the toners with high efficiency, thus improving the transfer efficiency. This reduces the quantity of the residual toners after image transfer and facilitates the collection of the residual toners in the developing device.
To achieve the foregoing objects, according to another aspect of the invention, an image forming apparatus comprises a rotary endless latent carrier; first means for charging the rotary endless latent carrier; second means for forming an electrostatic latent image on the rotary endless latent carrier; third means for developing the electrostatic latent image on the rotary endless latent carrier by supplying toners simultaneously with cleaning residual toners on the rotary endless latent carrier; fourth means having a rotary transfer roller for transferring the toners on the rotary endless latent carrier to a sheet by sandwiching the sheet between the rotary endless latent carrier and the rotary transfer roller; and fifth means for electrifying the rotary transfer roller.
An image forming method according to this aspect comprises a first step of charging a rotary endless latent carrier; a second step of forming an electrostatic latent image on the rotary endless latent carrier; a third step of developing the electrostatic latent image on the rotary endless latent carrier by supplying toners simultaneously with cleaning residual toners on the rotary endless latent carrier; and a fourth step of transferring the toners on the rotary endless latent carrier to a sheet by sandwiching the sheet between the rotary endless latent carrier and a rotary transfer roller to be electrified for transfer operation.
According to this aspect, the transfer step is performed by the transfer roller. In the conventional transfer involving a corona charger, charges supplied to the sheet by the corona discharger at a high humidity leak so that an electric field will not be applied to the toner image. This is because the supplied charges leak to the apparatus assembly through the sheet which has absorbed water in the air, dropping the electric resistance. Consequently, the transfer efficiency falls and increases the quantity of the residual toners after image transfer, making it difficult to collect the residual toners in the developing device. When the transfer roller is used, on the contrary, the transfer roller closely contact the sheet and serves as an electrode, permitting the charges to be supplied directly to the sheet. Even if some charges leak, the electric field does not decrease much. Further, since pressure and electrostatic force are used, a stable transfer efficiency can always be obtained regardless of a variation in environmental conditions. It is thus possible to reduce the quantity of the residual toners after image transfer, thus ensuring a cleanerless process which facilitates and stabilizes toner collection in the developing device.
To achieve the foregoing objects, according to a further aspect of the invention, an image forming apparatus comprises a rotary endless latent carrier; first means having a rotary charging brush for charging the rotary endless latent carrier; second means for forming an electrostatic latent image on the rotary endless latent carrier; third means for developing the electrostatic latent image on the rotary endless latent carrier by supplying toners simultaneously with cleaning residual toners on the rotary endless latent carrier; fourth means for transferring the toners on the rotary endless latent carrier to a sheet; and fifth means for supplying a charging voltage to the rotary charging brush.
An image forming method according to this aspect comprises a first step of charging a rotary endless latent carrier by a rotary charging brush applied with a charging voltage; a second step of forming an electrostatic latent image on the rotary endless latent carrier; a third step of developing the electrostatic latent image on the rotary endless latent carrier by supplying toners simultaneously with cleaning residual toners on the rotary endless latent carrier; and a fourth step of transferring the toners on the rotary endless latent carrier to a sheet.
According to this aspect, as the rotary endless latent carrier is charged by the charging brush, the charging brush contacts the residual toners on the latent carrier. Accordingly, the residual toners are charged through friction and supplied with charges from the charging brush. This causes the oppositely charged toners to be charged to a positive potential, thus ensuring toner collection in the developing step. As the charging brush is rotated to charge the latent carrier, the residual toners are paddled temporarily and the charging is carried out while putting the paddled toners again onto the latent carrier. Even if there are residual toners after image transfer, therefore, uneven charging can be prevented and the residual toners can be distributed, thus ensuring stable uniform charging. Other features and advantages of the present invention will become readily apparent from the following description taken in conjunction with the accompanying drawings.